The invention relates to a method for assembling a Pelton turbine wheel, to a bucket for such a wheel and to a wheel equipped with such a bucket.
Pelton turbines are conventionally used for transforming the kinetic energy of a fluid, such as the water of a waterfall, into mechanical energy. Such transformation is effected by rotating the turbine wheel under the effect of a tangential force exerted, on buckets arranged on the periphery of a rim, by a jet of water emerging from one or more injectors distributed around the wheel.
International Patent Application WO-A-99/49213 discloses using two annular flanges fast with the rim for supporting the buckets of a Pelton turbine wheel, which allows an optimalized distribution of the forces undergone by the buckets, the mechanical stresses in that case no longer being concentrated in a zone of fastening of the bucket on the rim. In this known device, each bucket is in abutment, by a convex surface, against radial arms of the flanges and immobilized with respect to each flange thanks to a locking screw.
The buckets of such a turbine wheel undergo a large number of impacts due to the jets of driving fluid coming from the injectors. The effect of these impacts is to apply each bucket alternately on the radial arms against which it is in abutment, which results in an alternate stressing of the materials constituting the buckets and the flanges. In addition, the surface abutment of the buckets on the flanges necessitates a precise adjustment of the outer dimensions of each bucket and of the corresponding housings provided in the flanges. Such an adjustment does not allow interchangeability of the buckets, which accordingly renders manufacture and maintenance of such a wheel complex.
It is a more particular object of the invention to overcome these drawbacks by proposing a method for assembling a wheel, as known from WO-A-99/49213, which makes it possible to avoid that the impacts of the jets of driving fluid generate alternate fatigue and/or stressing of the bucket with respect to the rim or to the flanges.
This method consists in:
pre-positioning, in a housing of a flange, a bucket supported by a part of its outer convex surface against an edge of this housing, providing a space between the convex surface of the outer radial part of this bucket and this edge;
producing an articulated linkage of this bucket on the rim and/or the flange on the inside of the bucket;
exerting on the outer radial part of the bucket a first calibrated force to bring it closer to the flange;
determining the distance between this outer radial part and this edge under this calibrated force;
slackening this calibrated force;
arranging, between this outer radial part and this edge, a wedge having a thickness substantially equal to the distance mentioned above, and
exerting on this outer radial part and maintaining a second force to bring it closer to the edge, this second force having an intensity greater than or equal to that of the first force.
Thanks to the invention, the bucket mounted on the wheel is subjected to a pre-stress formed by the second calibrated force which has the effect of immobilizing it firmly with respect to the flange and of compensating, with the wedge, the spacing existing between the outer radial part of the bucket and the flange. After establishment of the pre-stress, the wedge makes it possible to xe2x80x9cfilterxe2x80x9d the dynamic forces exerted on the bucket, particularly at the level of its outer radial part. The use of a wedge, of thickness determined as a function of the exact geometry of the bucket and of the flange and of the reaction of the bucket to the first calibrated force of bringing closer, makes it possible to envisage an interchangeability of the buckets, only the thickness of each wedge having to be adjusted precisely to the geometry of the surrounding parts.
According to a first advantageous aspect of the invention, the method consists in exerting the calibrated forces by means of a locking screw received in a housing made in the flange and penetrating in a tapping made in the outer radial part of the bucket. The calibrated force may thus be exerted by means of a torque wrench or any other controlled tightening means and have a considerable intensity, particularly in the case of a wheel of large diameter.
According to another advantageous aspect of the invention, the afore-mentioned first and second calibrated forces for bringing closer are substantially of the same intensity. Thus, when the wheel is in mounted configuration, each bucket is in abutment against the corresponding wedge under the effect of the calibrated force used for determining the thickness of this wedge. This assembly makes it possible to obtain a constant force in the locking screws by filtering the alternate forces due to the impacts of the jets of fluid.
The invention also relates to a turbine wheel bucket carrying out the method as described hereinbefore and, more specifically, to a bucket comprising a bi-concave surface for flow of a fluid for driving the wheel and a convex surface adapted to cooperate with at least one annular flange fast with a rim of the wheel. This bucket is characterized in that its convex surface forms at least one abutment zone located against the flange and at least one zone distant from the flange in an outer radial part of the bucket, this zone distant from the flange being adapted to be brought closer to a part of the flange located opposite by a calibrated force.
According to advantageous but non-obligatory aspects of the invention, this bucket incorporates one or more of the following characteristics:
The convex surface of the bucket also forms a zone distant from the flange in an inner radial part of the bucket. Thus, the localized abutment zone on the face is formed solely in a central part of this convex surface, which allows a certain displacement of the bucket in a radial direction.
The bucket comprises, at the level of the outer radial part and/or the inner radial part of its concave surface, at least one guide adapted to give to the flow of the afore-mentioned fluid a divergent component which is accentuated with respect to a median edge of the bucket. This guide makes it possible to avoid the fluid in the course of flow striking a radial arm of the flange disposed opposite the concave surface of the bucket. This guide may be provided to be adapted to direct the flow up to the level of a recess made in a radial arm of the flange disposed opposite this concave surface. This guide is advantageously formed by a rib extending, from an outer radial end zone of the bucket, in the direction of an inner radial part of the bucket, this rib bordering the outer notch of the bucket.
Finally, the invention relates to a Pelton turbine wheel which comprises a rim, at least one annular flange, fast with the rim and provided with housings for receiving buckets, and at least one bucket as described hereinbefore.
Such a wheel advantageously comprises, for this bucket or for each bucket, a wedge disposed between the afore-mentioned distant zone and the flange, the bucket or each of the buckets being subjected to a calibrated force of bringing this distant zone and the flange closer.
According to another advantageous aspect of the invention, the or each flange is provided with recesses for flow of the fluid driving the turbine from the concave surface of each bucket, the recesses being formed on an outer lateral surface of the flange.